Mac freezes after screensaver

When I stop using my mac for 15 minutes, my screensaver automatically turns on. The problem is that, sometimes, my computer freezes once I move the mouse and wont get out of screensaver; or it will get out of screensaver but sill freezes and the mouse wont move. I have to manually shut down the mac by holding the front button for a few seconds. It's starting to happen more and more often. Any suggestions on why? Or is there anyone else out there that has the same problem? I have Leopard 10.5.2